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How small can a population get before inbreeding becomes intolerable?

Explore the impact of small population sizes on inbreeding tolerance and genetic diversity. Learn about the effects of bottlenecks and founder events on population viability and conservation efforts.

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How small can a population get before inbreeding becomes intolerable?

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  1. How small can a population get before inbreeding becomes intolerable? Formulas first: If F = 1 and Ne = 4 M F 2Ne M + F Then F= 1 and F =1 + 1 2 4 M F8F 8M M + F Important! 60a

  2. How small can a population get before inbreeding becomes intolerable? F =1 + 1 8F 8M Research on domestic farm animals: natural selection for performance can balance inbreeding depression if the ΔF is no more than 1% per generation. So, F = 0.01 is a tolerable level of inbreeding 60a

  3. How small can a population get before inbreeding becomes intolerable? F =1 + 1 8F 8M If F = 0.01 is a tolerable level of inbreeding, then .01 = 1 + 1 so F = 25 and M = 25 8F 8M or, Ne = 50 Magic number! 60a

  4. What happens to the ‘magic number’ when sex ratios are unequal? 1 1 8Nm 8Nf F = + Conclusion: 15 = smallest number of effective individuals of one sex Number of females .005 tolerance 25 .01 tolerance 15 25 Number of males 60

  5. Population bottlenecks bottleneck Population size Time H = 1 - 1 = expected proportion of Ho retained after a 2Ne 1-generationbottleneck Ht = Ho 1 - 1 t = proportion of Ho retained t generations after 2Ne a bottleneck if Ne = 4 at t=0, then Ht=1 = 1 - 1 = 7 i.e. 1/8 of original H 2 x 4 8 was lost in 1 generation 61A

  6. The effect of bottlenecks on H Proportion of original heterozygosity remaining in small populations 61-1

  7. Probability of retaining a rare allele after a bottleneck of size N for a single generation 1.0 q = .10 q = .05 Probability of retention q = .01 0 0 25 Population size (Ne) 61-2

  8. Conclusions: Effects of Inbreeding and Bottlenecks  H P --------------------- ------------------- Bottleneck: decreases by 1/2Ne LARGE decrease with reproduction especially of rare alleles Inbreeding: LARGE decrease no change 65

  9. Effects of a genetic bottleneck in a plant: Lakeside daisy (Hymenoxys acaulis) - only one population survived agricultural development - self-incompatibility - requires mates of a different mating type -but alleles for only one mating type survived the bottleneck…..

  10. Founder Effect (Founders) Speke’s gazelle zoo Wild population Founders to zoo in 1970s: 1 male, 3 females N = 4, but Ne = 2 Initial severe inbreeding In 1982, N = 29 Inbreeding depression eliminated. Escaped F - vortex? 65A

  11. Contribution of 16 founders to the captive Guam Rail population 20 15 % Founder contribution 10 Expected 1/16 = .0625 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 31 32 33 36 98 Founders 65e

  12. Speke’s gazelle http://www.ultimateungulate.com/gazellespeke.html

  13. Okapi http://www.ultimateungulate.com/okapi.html

  14. Goeldi's monkey and Okapi • 40% loss of genetic diversity in captive populations • due to unequal founder contribution • Drosophila on Hawaiian Islands • 95 of 100 species are endemic to only one island or • volcano (results of migration of single, fertilized • “eves”)

  15. Population Management: Founder Effect founders Wild population Problem: Founding event--> forced inbreeding --> inbreeding depression Best Solution: MAXIMIZE GROWTH OF THE POPULATION A.S.A.P.!! 65A

  16. What is the minimum Nenecessary to maintainevolutionary potential??? 500 The other magic number Selection balances Drift…..Maybe!!!!

  17. The Biology of Small Populations • Population Dynamics of Small Populations • Genetics • Population Viability Assessment • The Ecology of Conservation and Extinction

  18. Minimum Viable Population • First stab: 50 / 500 rule. • John James Audubon – the first PVA

  19. Mauritian Kestrel 1970’s: Rarest bird in the world!

  20. Triage • Triage: Sorting the casualties of war into those too badly wounded to recover, those who can survive without help, and focusing on the remainder.

  21. Triage Conservation Biology • “We might abandon the Mauritius kestrel to its all-but-inevitable fate, and utilize the funds to proffer stronger support for any of the hundreds of threatened bird species that are more likely to survive” - Norman Myers, The Sinking Ark

  22. There are no hopeless cases, only people without hope and expensive ones – Michael Soulé Hopeless Cases?

  23. But . . . • The Mauritius kestrel was never exceptionally common • The fact still remains that we have limited resources to allocate to many species • 10 million $ to rent a Panda

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